The present invention relates generally to implantable medical interventional devices and methods for treating cardiac rhythm disorders, and more particularly to an implantable defibrillator for ventricular defibrillation, with pacing and sensing of the atrium and related methods of therapy using such implantable defibrillators.
Current implantable defibrillators perform a variety of functions designed to treat ventricular arrhythmias, including sensing of ventricular signals, detection of ventricular arrhythmias consisting of bradycardia, tachycardia, and fibrillation, and delivery of appropriate therapy automatically selected from among bradycardia and antitachycardia pacing, cardioverting and defibrillating shocks of the ventricles to correct the disorder. A serious problem with these devices is that a significant percentage of the defibrillating shocks delivered to the ventricles--about 25%--are falsely fired, delivered while the patient is fully conscious. The statistic is supported by recordings of cardiac activity among patients whose implanted devices have Holter function capabilities, and study of the recorded time period immediately before and up to delivery of the defibrillating or cardioverting shock, as well as by numerous interviews of defibrillator patients. Aside from the extreme pain suffered from a false shock, the patient tends to quickly lose confidence in the reliability of the implant as a life-saving device.
A large part of the reason for the false shocking is that many patients develop atrial fibrillation and atrial flutter spontaneously, and, with a tendency for fast conduction through the atrioventricular (AV) node, the ventricle is driven at a high rate. If the ECG criteria for ventricular tachycardia or fibrillation on which the implanted device relies for performing its therapy functions are fulfilled, a high energy cardioverting or defibrillating shock will be delivered to the ventricle. The shock--albeit false--is a proper response, given the criteria from which the determination was made. Rather, it is the data on which this response is based that is insufficient.
The solution to this problem of intermittent atrial fibrillation and flutter that can give rise to false shocks is to give greater attention to the status of the atrium. Currently available implantable defibrillator devices are unable to provide the solution because their focus is on the status of the ventricle. Recognition of atrial activity together with that of ventricular activity enables better discrimination of sinus rhythm, sinus tachycardia, ventricular fibrillation and ventricular flutter from one another. The better discrimination of the dysrhythmia--or absence thereof--allows the device to more properly respond with a corrective therapy that is based on the true condition of the patient. In other words, the device can better distinguish which heart chamber is attributable to the arrhythmia, so as to respond in kind.
It is a principal aim of the present invention to provide an implantable defibrillator that monitors the atrial status as well as the ventricular status, to discriminate arrhythmias of atrial origin from arrhythmias of ventricular origin, from which to better select the proper electrical therapy to be delivered to the patient's heart, and more specifically, to eliminate or at least substantially lessen the likelihood of false shocking.
Another problem which is not solved by the currently available spate of implantable defibrillators is the prominence of atrial arrhythmias which occur in implant patients because of a failure to address the atrial chamber. For example, the current devices perform ventricular pacing, but if retrograde conduction occurs the patient has a relatively high risk--40% or more--of developing atrial fibrillation. In contrast, patients who are experiencing constant atrial stimulation along with the ventricular pacing have a much lower risk--on the order of 5 to 10%--of developing intermittent or chronic atrial fibrillation.
Accordingly, another aim of the present invention is to provide an implantable defibrillator that performs pacing of the atrium as well as the ventricle, so as to enable better prevention of atrial arrhythmias.